1. Field of the Invention
The present invention relates to a pulsed light generator which is referred to a solid-state laser, a fiber laser, a fiber amplifier, or the like and has a function of amplifying a pulsed light using a semiconductor laser as a pumping light source.
2. Background Art
Conventionally, as a pumping light source for an amplification medium in some pulsed light generators such as solid-state lasers, fiber lasers, or fiber amplifiers, a semiconductor laser (laser diode: LD) that outputs light from a fiber has been used.
For an LD as a pumping light source (hereinafter referred to as a pumping LD), for example, a shunt type constant current driving circuit as shown in FIG. 6 is often used, and in that case, a constant output power is maintained continually with the pumping LD.
A basic shunt type driving circuit shown in FIG. 6 is composed of a shunt resistor 52, a comparator 53 such as an OP amplifier, and a transistor 54.
In this shunt type driving circuit, a voltage signal is input to the comparator 53, and an output of the comparator 53 is input to the transistor 54 (a base side).
An output on a collector side of the transistor 54 returns to the comparator 53 with a potential pattern made by the shunt resistor.
If an output on an emitter side of the transistor 54 is applied to the pumping LD 3 as a driving current, an optical output corresponding to the driving current is obtained from the pumping LD 3.
The shunt type driving circuit described above is advantageous to keep the current at a constant value, but is unable to change at high speed between a state where the pumping LD 3 is laser-oscillated (hereinafter, ON state) and a state where the pumping LD 3 is not laser-oscillated (OFF state).
On the other hand, as represented by a switching power supply, a constant voltage circuit adopting a switching element has generally been used.
The reason is that it is easy to minimize losses in a device in which a power circuit or a power supply circuit uses the constant voltage circuit adopting the switching element.
A technology to adopt the power circuit or the power supply circuit in a pumping LD of an optical amplifier is also known.
In the specification of U.S. Pat. No. 5,325,383 and the specification of U.S. Pat. No. 5,283,794, a technology to perform pulse driving of a pumping LD using pulse width modulation (PWM) is disclosed.
An LD driving method that makes the start of an output pulse fast by driving an LD through correlation between a phase of a switching frequency and a phase of a pulse frequency is disclosed in some technical literatures (for example, see specifications of U.S. Pat. No. 5,867,305, U.S. Pat. No. 5,933,271, and U.S. Pat. No. 6,081,369).
Theses literatures disclose a technology to intermittently drive the pumping LD at a time that is shorter than the excitation life of the amplification medium in order to remove noise (for example, amplified spontaneous emission (ASE) light or light of Rayleigh scattering) that limits a gain or peak power in the pulse amplification.
Furthermore, a technology to improve a gain through synchronizing the frequency of the pumping pulse light in the pumping light source that outputs a pulse laser (amplifier of the pulsed light) with the signal frequency is already known.
Since the pulsed light generator using the above-described LD as the pumping light source can obtain an output light that has high peak power, it is frequently used for laser processing or laser measurement.
In a laser processing device or a laser measurement device, however, if the pulsed light that is emitted from the high power pulsed light generator is reflected from the surface of an irradiated body or from a position that is near the surface of the irradiated body, the situation frequently occurs in which the pulsed light emitted from an output port of the pulsed light generator becomes incident light to the device in the direction opposite to the emission direction of the output pulsed light and returns to the device and is propagated to the optical amplifier.
Moreover, part of the reflected light that is reflected from the irradiated body reaches the pumping LD of the optical amplifier, and corresponding to the driving state of the pumping LD, may damage the pumping LD.
In the conventional technique that appears in the above-described literature, the problem that the pumping LD is damaged by the reflected light is not particularly considered, and thus, conventionally, damage to the pumping LD due to the reflected light, which is reflected from the irradiated body and is incident into the high power pulsed light generator, cannot be avoided.
As described above, the conventional high power pulsed light generator using the LD as the pumping light source has the problem that the light which is reflected by the irradiated body and is transferred into the device to be incident to the optical amplifier (hereinafter called a “returning light from the irradiated body”) reaches the pumping LD that is in a laser oscillation state to damage the pumping LD.